1. Field of the Invention
The present invention relates to a wavelength dispersion compensation system for compensating even for higher-order dispersion.
2. Description of the Related Art
In optical communications, there is a great demand for a wavelength dispersion compensator for solving the problem that when an optical pulse is transmitted over a long distance, the signal transmitted over a long distance degrades.
FIG. 1 shows a conventional fiber-optical communications system for transmitting information by light.
In FIG. 1, a transmitter 1 transmits a pulse to a light receiver 6 through an optical fiber 2. However, the wavelength dispersion, which is also called xe2x80x9cchromatic dispersionxe2x80x9d, of the optical fiber degrades the signals of the system. More specifically, the propagation velocity of a signal in an optical fiber depends on the wavelength of the signal due to wavelength dispersion. For example, the phenomenon that a pulse with a long wavelength (for example, a pulse 3 with a wavelength representing a red color pulse) propagates faster than a pulse with a short wavelength (for example, a pulse 4 with a wavelength representing a blue color pulse) is generally called xe2x80x9cnormal dispersionxe2x80x9d. Conversely, the phenomenon that a pulse with a short wavelength (for example, a blue color pulse) propagates faster than a pulse with a long wavelength (for example, a red color pulse) is generally called xe2x80x9cabnormal dispersionxe2x80x9d. Therefore, in the case that a pulse 5, including both red and blue color pulses is transmitted from the transmitter 1, the pulse 5 is separated into the red and blue color pulses when the pulse 5 is transmitted through the optical fiber 2. The light receiver 6 receives the separated red and blue color pulses at respective different times. In this case, the case where the red color pulse propagates faster than the blue color pulse is xe2x80x9cnormal dispersionxe2x80x9d. For an example of other pulse propagation, if a pulse has consecutive wavelength components ranging from blue to red, the pulse is expanded in an optical fiber since the red and blue components propagate at different speeds, and is distorted due to the wavelength dispersion. Since all the pulses include limited numbers of wavelength range, such wavelength dispersion often occurs in fiber-optical communications. Therefore, to obtain a high transmitting power in a fiber-optical communications system, wavelength dispersion must be compensated for.
To compensate for chromatic dispersion, a diffraction grating pair, chirp fiber grating, dispersion compensation fiber and the like has traditionally been used to compensate for chromatic dispersion, in particular, the diffraction grating pair being not only for chromatic dispersion in a fiber. Japanese Patent Laid-open Nos. 10-534450 and 11-513133 have proposed a device, including a xe2x80x9cVirtually Imaged Phased Arrayxe2x80x9d (VIPA) as an inverted-dispersion component.
FIGS. 2A and 2B show the operation of a VIPA.
As shown in FIG. 2A, input light, including a plurality of wavelength components are collected by a cylindrical lens and are inputted to a VIPA plate. The inputted light is reflected a umber of times on reflection films provided on the each side of the VIPA while expanding. Light is outputted in small amounts from the VIPA plate during multi-reflection. The plurality of outputted light interfere each other and generate a plurality of parallel light rays propagate in different directions for each wavelength. The operation of this VIPA plate is understood as shown in FIG. 2B. In other words, since light is reflected a multiple number of times on a plurality of reflection planes on a VIPA plate, it can be virtually considered that light is outputted from a plurality of virtual images. Specifically, a plurality of light a to e are linearly arrayed. All the distances between the two virtual images are equal. Since it can be virtually considered that virtual images are phase-matched and arrayed, the component is named as a VIPA plate.
FIG. 3 shows a dispersion compensation system using a transmission type diffraction grating with the function equivalent to a VIPA plate.
How to compensate for dispersion is described later with reference to a VIPA plate. The operation of a transmission type diffraction grating is described here.
In a transmission type diffraction grating, stair-type steps are formed on the surface of a transparent material, and a plurality of light a to e are outputted from the surfaces of these steps. Since the plurality of light a to e are outputted after being transmitted over distances 1 to 5, respectively, there are phase differences between the plurality of light a to e after emission. After emission the plurality of outputted light a to e interfere with each other and generate a plurality of parallel light propagating in different directions for each wavelength. In a transmission type diffraction grating, a plurality of light a to e directly transmitted from a real light source are arrayed, while in a VIPA plate, a plurality of light a to e transmitted from virtual images are arrayed. However, the substantial operations of both the transmission type diffraction grating and VIPA plate are the same.
FIGS. 4 and 5 show the operational principle of a dispersion compensator using a VIPA plate.
FIG. 4 shows the appearance of a dispersion compensator using a VIPA plate. A plurality of light inputted from an optical fiber are converted into a plurality of parallel light by a collimation lens and are collected on a VIPA plate by a cylindrical lens. As described earlier, in the VIPA plate, a plurality of parallel light propagating in different directions for each wavelength are generated and outputted. The plurality of output light are collected on a mirror by a lens. The plurality of light inputted to the mirror are reflected in different directions and are inputted to the lens again. The plurality of light inputted to the lens are converted into a plurality of parallel light, are returned to the VIPA plate and are outputted to the cylindrical lens. The cylindrical lens generates a plurality of parallel light and inputs the plurality of parallel light to the collimation lens. The collimation lens collects the plurality of inputted parallel light and combines the plurality of parallel light in an optical fiber.
FIG. 5 shows a mechanism for compensating for wavelength dispersion by the configuration shown in FIG. 4.
As described earlier, a plurality of light outputted from a VIPA plate can be virtually regarded as a plurality of light outputted from virtual images a to d. For example, light outputted from a virtual image a (in this case, a real image) propagates toward a lens, is defracted by the lens and hits the surface of a mirror. The mirror reflects the inputted light in a different direction and inputs the light at a different position of the lens. The lens converts the inputted light into parallel light and returns to the VIPA plate. In this case, the light returned to the VIPA plate is inputted to a position different from the position from which the light is outputted initially. As shown in FIG. 5, it can be virtually considered that the light is returned to a different virtual image c. However, since virtual images a and c are separate from each other, the light is transmitted additional by distance 1 and is returned. Therefore, such transmitted light is returned with a delay caused by the extra transmission distance. Therefore, if a blue color pulse is delayed from a red color pulse due to wavelength dispersion, the propagation delay between the red and blue color pulses can be avoided by configuring the system so that the red pulse is transmitted along the route shown in FIG. 5 and delaying the propagation of the red pulse. The propagation time can also be reduced by configuring the system so that the light route is short. Therefore, if a blue color pulse propagates faster than a red color pulse, the propagation delay can also be avoided by making the transmission distance of the red color pulse in a dispersion compensator shorter than that of the blue color pulse. Thus, dispersion can be compensated.
However, there is also a special need for practical inverted-dispersion component (wavelength dispersion compensator) to be used in a wavelength-division multiplex transmission system. The wavelength dispersion of a general commercially available optical fiber is not constant, but is often positively inclined (the longer a wavelength, the greater the wavelength dispersion). For example, in the case of a general single-mode fiber (SMF), wavelength dispersion per kilometer and wavelength dispersion slope per kilometer are +17 ps/nm and 0.06 ps/nm2, respectively. If a necessary wavelength bandwidth is, for example, 35 nm, the wavelength dispersion changes approximately +2 ps/nm. Such an inclination of a wavelength dispersion slope is sometimes called xe2x80x9cwavelength dispersion slopexe2x80x9d or xe2x80x9csecondary wavelength dispersionxe2x80x9d. The wavelength dispersion slope is not always positive (the longer a wavelength, the grater the wavelength dispersion). In a dispersion shifted fiber where zero dispersion wavelength is shifted to a wavelength band of 1.5 xcexcm by modifying the structure dispersion of a fiber, there can be a negative wavelength dispersion slope in a wavelength band longer than the zero dispersion wavelength. In the actual wavelength dispersion of an optical fiber transmission line, both the wavelength dispersion per unit length and wavelength dispersion slope are determined by the type of optical fiber, and then both the actual wavelength dispersion and wavelength dispersion slope are determined by the length (transmission distance) of the optical fiber. To compensate for the wavelength dispersion of such an actual optical fiber transmission line by an inverted-dispersion component, first it is preferable to be able to modify a wavelength dispersion amount by some amount by a wavelength dispersion compensator. This is because both the type and transmission distance of an optical fiber vary depending on the transfer rate and wavelength band of a transmission system, the installation time of an optical fiber and the situation of the installation site. Furthermore, in the case of a wavelength-division multiplex transmission, it is not sufficient to compensate for only wavelength dispersion, as described earlier, and wavelength dispersion slope is also a problem. This is because even if dispersion can be compensated for in the wavelength of a specific signal channel, wavelength dispersion cannot be completely compensated for in the wavelength of a different signal channel if the wavelength dispersion of an inverted-dispersion component is constant. For this reason, it is preferable for an inverted-dispersion component for a wavelength-division multiplex transmission to have a wavelength dispersion slope. Furthermore, since a transmission distance varies, as described earlier, the wavelength dispersion slope changes in proportion to the length along with the wavelength dispersion. Therefore, it is preferable to be also able to modify the wavelength dispersion slope by some order. However, the value to be given to the wavelength dispersion slope is not uniquely determined by the value of the wavelength dispersion. This is because not only the wavelength dispersion, but also the wavelength dispersion slope vary depending on the type of an optical fiber.
Specifically, if in a wavelength-division multiplex transmission, the wavelength dispersion of an optical fiber transmission line is compensated for by an inverted-dispersion component, it is preferable to independently modify both the wavelength dispersion and wavelength dispersion slope. However, this method is not specifically described in Japanese Patent Laid-open Nos. 10-534450 and 11-513133. This cannot be implemented by the conventional inverted-dispersion component. First, although an index profile with an inverted-dispersion slope is available for a dispersion-compensated fiber, the method is not practical since both a variety of index profiles and a variety of lengths are needed to modify the value. As described earlier, the cost is high, loss is great and the size is excessive. In a chirp fiber grating, if the chirp of a fiber grating is optimally designed, an inverted-dispersion slope can be provided. However, since both a variety of index profiles and a variety of lengths are needed to modify the value, the method is not practical. Even if temperature is modified, both the wavelength dispersion and wavelength dispersion slope cannot be independently modified. As described earlier, a wavelength band sufficient to compensate for light, including many wavelengths, such as multi-wavelength light cannot be obtained. In the conventional diffraction grating, wavelength dispersion and a wavelength dispersion slope may be independently modified depending on the combination method of diffraction gratings. However, since as described earlier, in a fiber-optical communications system, a practical size cannot provide dispersion sufficient to compensate for a fairly large amount of chromatic dispersion caused in the fiber-optical communications system, the conventional diffraction grating is not practical. If the bit rate is further increased, compensation by the wavelength dispersion slope described above, that is, compensation by higher-order dispersion, such as tertiary dispersion may have to be made.
It is an object of the present invention to provide a wavelength dispersion compensation system for compensating for both wavelength dispersion and other higher-order wavelength dispersions, including the wavelength dispersion slope, that are simultaneously accumulated in an optical fiber in the multi-wavelength range of a multi-channel.
The wavelength dispersion compensation system of the present invention is used to compensate for wavelength dispersion which is suffering by light when being transmitted through an optical fiber. The wavelength dispersion compensation system comprises a plurality of wavelength dispersion compensation units compensating for the wavelength dispersion of each order and a connection unit sequentially passing the light through the plurality of wavelength dispersion compensation units.
According to the present invention, the wavelength dispersion of each order is compensated for by connecting each wavelength dispersion compensation units designed to compensate for the wavelength dispersion of each order and sequentially passing light which has suffered from wavelength dispersion through the wavelength dispersion compensation units. Thus, the wavelength of any order can be compensated for. In particular, by replacing the wavelength dispersion compensator with a module for simply returning light without wavelength dispersion, a wavelength dispersion compensation system for compensating for only the wavelength dispersion of a order to be compensated for can be flexibly configured without the modification of the entire configuration of the wavelength dispersion compensation system.